The cell cycle is the sequence of events leading to DNA duplication and cell division. In eukaryotes, thousands of proteins and factors regulate cell cycle progression, allowing the cell to stop if certain conditions are not met. In my thesis I focused my attention on one of these regulators: the serine/threonine kinase Plk1. The aim of my PhD was to dissect the dynamics of Plk1 in mitotic entry and during stress response. In the first part of my PhD I investigated the mechanism of Plk1 activation in late G2. Plk1 promotes mitotic entry triggering Cdk1 activation. Using a combination of biochemistry and cell biology, we demonstrated in HeLa cells that the Cdk1-dependent phosphorylation of Bora on three conserved sites is required to activate Plk1 in a G2-checkpoint recovery assay. Altogether, our data demonstrate both in C. elegans and human cells that Cdk1 dependent Bora phosphorylation is important to activate Plk1 during mitotic entry. In the second part of my PhD, I focused on the role of Plk1 in oxidative stress response. Using HeLa cells and a derived cell line in which endogenous Plk1 was tagged with eGFP, I observed that Plk1 localized to microtubules after arsenite-induced oxidative stress. To further investigate the role of Plk1 in oxidative stress response I focused on Plk1 interactors involved in oxidative stress. The protein UBAP2L, co-immunoprecipitates with Plk1 and it is required for stress granules formation in arsenite treated cells. My work demonstrated that UBAP2L is a cell cycle regulated protein, but also that is not required for cell cycle progression in HeLa cells. I observed the formation of Plk1-eGFP granules in mitotic cells in which UBAP2L was depleted. Investigating the nature of these granules, I showed that UBAP2L colocalizes with stress granules and it is required for stress granules formation. Overall, my thesis contributed to the understanding of Plk1 activation and roles, both in cell cycle progression and stress response.